U.S. patent application number 10/444322 was filed with the patent office on 2004-02-12 for high gain antenna for wireless applications.
Invention is credited to Chiang, Bing, Lynch, Michael James, Wood, Douglas Harold.
Application Number | 20040027304 10/444322 |
Document ID | / |
Family ID | 33489343 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040027304 |
Kind Code |
A1 |
Chiang, Bing ; et
al. |
February 12, 2004 |
High gain antenna for wireless applications
Abstract
An antenna having a central active element and a plurality of
passive dipoles surrounding the active element is disclosed. The
passive dipoles increase the antenna gain by increasing the
radiated energy in the azimuth direction. In another embodiment a
plurality of parasitic directing elements extend radially outward
from the passive dipoles.
Inventors: |
Chiang, Bing; (Melbourne,
FL) ; Lynch, Michael James; (Merritt Island, FL)
; Wood, Douglas Harold; (Palm Bay, FL) |
Correspondence
Address: |
BEUSSE BROWNLEE WOLTER MORA & MAIRE, P. A.
390 NORTH ORANGE AVENUE
SUITE 2500
ORLANDO
FL
32801
US
|
Family ID: |
33489343 |
Appl. No.: |
10/444322 |
Filed: |
May 23, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10444322 |
May 23, 2003 |
|
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09845133 |
Apr 30, 2001 |
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6606057 |
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Current U.S.
Class: |
343/810 ;
343/817; 343/818 |
Current CPC
Class: |
H01Q 9/32 20130101; H01Q
3/242 20130101; H01Q 3/446 20130101; H01Q 3/2641 20130101; H01Q
1/246 20130101; H01Q 15/02 20130101; H01Q 19/32 20130101; H01Q
13/28 20130101; H01Q 21/205 20130101; H01Q 3/24 20130101 |
Class at
Publication: |
343/810 ;
343/817; 343/818 |
International
Class: |
H01Q 021/00; H01Q
019/10 |
Claims
What is claimed is:
1. An antenna comprising: an active element; a plurality of passive
dipoles spaced apart from and circumscribing the active element;
and a controller for selectably controlling the passive dipoles to
operate in a reflective or a directive mode.
2. The antenna of claim 1 wherein the antenna directivity is
increased along a longitudinal plane through the active
element.
3. The antenna of claim 1 wherein antenna radiation is attenuated
in a direction perpendicular to the longitudinal plane through the
active element.
4. The antenna of claim 1 wherein the controller modifies an
effective electrical length of one or more of the plurality of
passive dipoles to effectuate the reflective or the directive
mode.
5. The antenna of claim 1 wherein each one of the plurality of
passive dipoles comprises an upper segment and a lower segment.
6. The antenna of claim 5 wherein the controller modifies an
effective electrical length of the upper segment of one or more of
the plurality of passive dipoles to effectuate the reflective or
the directive mode.
7. The antenna of claim 6 wherein the controller comprises a
switching element connected between the upper segment and ground
for introducing an impedance between the upper segment and
ground.
8. The antenna of claim 7 wherein the switching element selectably
introduces one of a first and a second impedance between the upper
segment and ground, wherein the first impedance comprises an
inductance and the second impedance comprises a capacitance.
9. The antenna of claim 1 further comprising a ground plane
proximate a lower end of the active element, wherein the lower
segment is formed from a region of the ground plane.
10. The antenna of claim 1 wherein a received or a transmitted
signal frequency is the carrier frequency in a wireless system
operating according to one of the following standards,
code-division multiple access, time division multiple access, IEEE
802.11, Bluetooth, and global system for mobile communications.
11. The antenna of claim 1 wherein the active element and the
plurality of passive dipoles are vertically oriented.
12. The antenna of claim 1 wherein the plurality of passive dipoles
are radially spaced apart from the active element.
13. The antenna of claim 1 wherein the plurality of passive dipoles
are radially spaced an equal distance from the active element.
14. The antenna of claim 1 further comprising a ground, wherein the
active element and the plurality of passive dipoles comprise
vertically disposed rectangular conductors, wherein each one of the
plurality of passive dipoles further comprises an upper and a lower
segment, and wherein an upper end of the lower segment is connected
to the ground.
15. The antenna of claim 1 further comprising a ground, wherein
each one of the plurality of passive dipoles comprises an elongated
conductive upper segment switchably connected to the ground and a
lower segment in substantial vertical alignment with the upper
segment, and wherein an upper end of the lower segment is
contiguous with a vertically disposed ground plane extending
radially inward in the direction of the active element.
16. The antenna of claim 15 wherein the upper segment is switchably
connected to the ground through an impedance.
17. The antenna of claim 1 wherein each one of the plurality of
passive dipoles has a physical length, and wherein the antenna
transmits or receives an operating signal having a wavelength, and
wherein the physical wavelength is less than about a
wavelength.
18. The antenna of claim 17 wherein the operating signal comprises
a plurality of operating signals, and wherein frequencies of the
plurality of operating signals are harmonically related.
19. The antenna of claim 1 further comprising a plurality of
parasitic gratings spaced apart from and circumscribing the active
element.
20. The antenna of claim 19 wherein each one of the plurality of
parasitic gratings is radially aligned with one of the plurality of
passive dipoles.
21. The antenna of claim 19 wherein each one of the plurality of
parasitic gratings is disposed between two adjacent ones of the
plurality of passive dipoles.
22. The antenna of claim 19 wherein the plurality of parasitic
gratings are arranged in one or more concentric circles from the
active element.
23. The antenna of claim 19 wherein a length of each one of the
plurality of parasitic gratings is less than about one-half
wavelength at an operating frequency of the antenna.
26. The antenna of claim 19 wherein each one of the plurality of
parasitic gratings is vertically oriented.
27. The antenna of claim 19 further comprising a ground, wherein
each one of the plurality of parasitic gratings comprises an
elongated conductive element shorted to the ground.
28. The antenna of claim 19 further comprising a ring structure for
supporting the plurality of parasitic elements.
29. The antenna of claim 28 wherein the ring structure is removably
positioned outwardly from and concentric with the plurality of
passive dipoles.
30. The antenna of claim 1 further comprising a ground plane
surrounding the active element.
31. The antenna of claim 30 wherein the ground plane comprises a
substantially horizontal ground plane, and wherein each one of the
plurality of passive dipoles comprises an elongated conductive
upper segment switchably connected to the horizontal ground plane
and a lower segment in substantial vertical alignment with the
upper segment, and wherein an upper end of the lower segment is
contiguous with a substantially vertical ground plane extending
radially inward in the direction of the active element, and wherein
the substantially horizontal ground plane is connected to the
substantially vertical ground plane.
32. The antenna of claim 1 further comprising a plurality of planar
ground structures disposed radially outwardly from the active
element, wherein each one of the plurality of passive dipoles
comprises an elongated conductive upper segment switchably
connected to the ground and a lower segment in substantial vertical
alignment with the upper segment, and wherein the lower segment is
formed from one of the plurality of planar ground structures.
Description
[0001] This patent application is a continuation-in-part of the
patent application entitled High Gain Planar Scanned Antenna Array,
filed on April 30, 2001, and assigned application Ser. No.
09/845,133.
FIELD OF THE INVENTION
[0002] This invention relates to mobile or portable cellular
communication systems and more particularly to an antenna apparatus
for use in such systems, wherein the antenna apparatus offers
improved beam-forming capabilities by increasing the antenna gain
in the azimuth direction.
BACKGROUND OF THE INVENTION
[0003] Code division multiple access (CDMA) communication systems
provide wireless communications between a base station and one or
more mobile or portable subscriber units. The base station is
typically a computer-controlled set of transceivers that are
interconnected to a land-based public switched telephone network
(PSTN). The base station further includes an antenna apparatus for
sending forward link radio frequency signals to the mobile
subscriber units and for receiving reverse link radio frequency
signals transmitted from each mobile unit. Each mobile subscriber
unit also contains an antenna apparatus for the reception of the
forward link signals and for the transmission of the reverse link
signals. A typical mobile subscriber unit is a digital cellular
telephone handset or a personal computer coupled to a cellular
modem. In such systems, multiple mobile subscriber units may
transmit and receive signals on the same center frequency, but
different modulation codes are used to distinguish the signals sent
to or received from individual subscriber units.
[0004] In addition to CDMA, other wireless access techniques
employed for communications between a base station and one or more
portable or mobile units include time division multiple access
(TDMA), the global system for mobile communications (GSM), the
various 802.11 standards described by the Institute of Electrical
and Electronics Engineers (IEEE) and the so-called "Bluetooth"
industry-developed standard. All such wireless communications
techniques require the use of an antenna at both the receiving and
transmitting end. Any of these wireless communications techniques,
as well as others known in the art, can employ one or more antennas
constructed according to the teachings of the present invention.
Increased antenna gain, as taught by the present invention, will
provide improved performance for all wireless systems.
[0005] The most common type of antenna for transmitting and
receiving signals at a mobile subscriber unit is a monopole or
omnidirectional antenna. This antenna consists of a single wire or
antenna element that is coupled to a transceiver within the
subscriber unit. The transceiver receives reverse link audio or
data for transmission from the subscriber unit and modulates the
signals onto a carrier signal at a specific frequency and
modulation code (i.e., in a CDMA system) assigned to that
subscriber unit. The modulated carrier signal is transmitted by the
antenna. Forward link signals received by the antenna element at a
specific frequency are demodulated by the transceiver and supplied
to processing circuitry within the subscriber unit.
[0006] The signal transmitted from a monopole antenna is
omnidirectional in nature. That is, the signal is sent with
approximately the same signal strength in all directions in a
generally horizontal plane. Reception of a signal with a monopole
antenna element is likewise omnidirectional. A monopole antenna
alone cannot differentiate a signal received in one azimuth
direction from the same or a different signal coming from another
azimuth direction. Also, a monopole antenna does not produce
significant radiation in the zenith direction. The antenna pattern
is commonly referred to as a donut shape with the antenna element
located at the center of the donut hole.
[0007] A second type of antenna that may be used by mobile
subscriber units is described in U.S. Pat. No. 5,617,102. The
system described therein provides a directional antenna system
comprising two antenna elements mounted on the outer case of a
laptop computer, for example. The system includes a phase shifter
attached to each element. The phase shifters impart a phase angle
delay to the signal input thereto, thereby modifying the antenna
pattern (which applies to both the receive and transmit modes) to
provide a concentrated signal or beam in a selected direction.
Concentrating the beam is referred to as an increase in antenna
gain or directivity. The dual element antenna of the cited patent
thereby directs the transmitted signal into predetermined sectors
or directions to accommodate for changes in orientation of the
subscriber unit relative to the base station, thereby minimizing
signal losses due to the orientation change. The antenna receive
characteristics are similarly effected by the use of the phase
shifters.
[0008] CDMA cellular systems are recognized as interference limited
systems. That is, as more mobile or portable subscriber units
become active in a cell and in adjacent cells, frequency
interference increases and thus bit error rates also increase. To
maintain signal and system integrity in the face of increasing
error rates, the system operator decreases the maximum data rate
allowable for one or more users, or decreases the number of active
subscriber units, which thereby clears the airwaves of potential
interference. For instance, to increase the maximum available data
rate by a factor of two, the number of active mobile subscriber
units can be decreased by one half. However, this technique is not
typically employed to increase data rates due to the lack of
priority assignments for individual system users. Finally, it is
also possible to avert excessive interference by using directive
antennas at both (or either) the base station and the portable
units.
[0009] Generally, a directive antenna beam pattern can be achieved
through the use of a phased array antenna. The phased array is
electronically scanned or steered to the desired direction by
controlling the phase of the input signal to each of the phased
array antenna elements. However, antennas constructed according to
these techniques suffer decreased efficiency and gain as the
element spacing becomes electrically small compared to the
wavelength of the transmitted or received signal. When such an
antenna is used in conjunction with a portable or mobile subscriber
unit, the antenna array spacing is relatively small and thus
antenna performance is correspondingly compromised.
[0010] Various disadvantages are inherent in prior art antennas
used on mobile subscriber units in wireless communications systems.
One such problem is called multipath fading. In multipath fading, a
radio frequency signal transmitted from a sender (either a base
station or mobile subscriber unit) may encounter interference in
route to the intended receiver. The signal may, for example, be
reflected from objects, such as buildings, thereby directing a
reflected version of the original signal to the receiver. In such
instances, the receiver receives two versions of the same radio
signal; the original version and a reflected version. Each received
signal is at the same frequency, but the reflected signal may be
out of phase with the original signal due to the reflection and
consequent differential transmission path length to the receiver.
As a result, the original and reflected signals may partially or
completely cancel each other (destructive interference), resulting
in fading or dropouts in the received signal, hence the term
multipath fading.
[0011] Single element antennas are highly susceptible to multipath
fading. A single element antenna has no way of determining the
direction from which a transmitted signal is sent and therefore
cannot be turned to more accurately detect and receive a signal in
any particular direction. Its directional pattern is fixed by the
physical structure of the antenna. Only the antenna physical
position or orientation (e.g., horizontal or vertical) can be
changed in an effort to obviate the multipath fading effects.
[0012] The dual element antenna described in the aforementioned
reference is also susceptible to multipath fading due to the
symmetrical and opposing nature of the hemispherical lobes formed
by the antenna pattern when the phase shifter is activated. Since
the lobes created in the antenna pattern are more or less
symmetrical and opposite from one another, a signal reflected
toward the backside of the antenna (relative to a signal
originating at the front side) can be received with as much power
as the original signal that is received directly. That is, if the
original signal reflects from an object beyond or behind the
intended receiver (with respect to the sender) and reflects back at
the intended receiver from the opposite direction as the directly
received signal, a phase difference in the two signals creates
destructive interference due to multipath fading.
[0013] Another problem present in cellular communication systems is
inter-cell signal interference. Most cellular systems are divided
into individual cells, with each cell having a base station located
at its center. The placement of each base station is arranged such
that neighboring base stations are located at approximately
sixty-degree intervals from each other. Each cell may be viewed as
a six-sided polygon with a base station at the center. The edges of
each cell abut and a group of cells form a honeycomb-like image if
each cell edge were to be drawn as a line and all cells were viewed
from above. The distance from the edge of a cell to its base
station is typically driven by the minimum power required to
transmit an acceptable signal from a mobile subscriber unit located
near the edge of the cell to that cell's base station (i.e., the
power required to transmit an acceptable signal a distance equal to
the radius of one cell).
[0014] Intercell interference occurs when a mobile subscriber unit
near the edge of one cell transmits a signal that crosses over the
edge into a neighboring cell and interferes with communications
taking place within the neighboring cell. Typically, signals in
neighboring cells on the same or closely spaced frequencies cause
intercell interference. The problem of intercell interference is
compounded by the fact that subscriber units near the edges of a
cell typically employ higher transmit powers so that their
transmitted signals can be effectively received by the intended
base station located at the cell center. Also, the signal from
another mobile subscriber unit located beyond or behind the
intended receiver may arrive at the base station at the same power
level, causing additional interference.
[0015] The intercell interference problem is exacerbated in CDMA
systems, since the subscriber units in adjacent cells typically
transmit on the same carrier or center frequency. For example,
generally, two subscriber units in adjacent cells operating at the
same carrier frequency but transmitting to different base stations
interfere with each other if both signals are received at one of
the base stations. One signal appears as noise relative to the
other. The degree of interference and the receiver's ability to
detect and demodulate the intended signal is also influenced by the
power level at which the subscriber units are operating. If one of
the subscriber units is situated at the edge of a cell, it
transmits at a higher power level, relative to other units within
its cell and the adjacent cell, to reach the intended base station.
But, its signal is also received by the unintended base station,
i.e., the base station in the adjacent cell. Depending on the
relative power level of two same-carrier frequency signals received
at the unintended base station, it may not be able to properly
differentiate a signal transmitted from within its cell from the
signal transmitted from the adjacent cell. There is required a
mechanism for reducing the subscriber unit antenna's apparent field
of view, which can have a marked effect on the operation of the
forward link (base to subscriber) by reducing the number of
interfering transmissions received at a base station. A similar
improvement in the reverse link antenna pattern allows a reduction
in the desired transmitted signal power, to achieve a receive
signal quality.
BRIEF SUMMARY OF THE INVENTION
[0016] An antenna according to the present invention comprises an
active element and a plurality of passive dipoles spaced apart from
and circumscribing the active element. A controller selectably
controls the passive dipoles to operate in a reflective or a
directive mode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The foregoing and other features and advantages of the
invention will be apparent from the following description of the
preferred embodiments of the invention, as illustrated in the
accompanying drawings in which like referenced characters refer to
the same parts throughout the different figures. The drawings are
not necessarily to scale, emphasis instead being placed upon
illustrating the principles of the invention.
[0018] FIG. 1 illustrates a cell of a CDMA cellular communication
system.
[0019] FIGS. 2 and 3 illustrate antenna structures for increasing
antenna gain to which the teachings of the present invention can be
applied.
[0020] FIG. 4 illustrates an antenna array wherein each antenna has
a variable reactive load.
[0021] FIGS. 5 and 6 illustrate the use of a dielectric ring in
conjunction with the present invention.
[0022] FIGS. 7 and 8 illustrate a corrugated ground plane for
producing a more directive antenna beam in accordance with the
teachings of the present invention.
[0023] FIGS. 9, 10, 11, 12 and 13 illustrate an embodiment of the
present invention including vertical gratings.
[0024] FIG. 15 illustrates another antenna constructed according to
the teachings of the present invention.
[0025] FIG. 16 illustrates a top view of the antenna of FIG.
15.
[0026] FIG. 17 illustrates a side view of one element of the
antenna of FIG. 15.
[0027] FIG. 18 illustrates a switch for use with the antenna of
FIG. 15.
[0028] FIG. 19 illustrates a side view of an alternative embodiment
of the element of FIG. 17.
[0029] FIG. 20 illustrates a perspective view of yet another
antenna constructed according to the teachings of the present
invention.
[0030] FIGS. 21A-21D illustrate various antenna element shapes for
use with an antenna constructed according to the teachings of the
present invention.
[0031] FIG. 22 illustrates another antenna constructed according to
the teachings of the present invention.
[0032] FIGS. 23 and 24 illustrate elements of the antenna of FIG.
22.
DETAILED DESCRIPTION OF THE INVENTION
[0033] FIG. 1 illustrates one cell 50 of a typical CDMA cellular
communication system. The cell 50 represents a geographical area in
which mobile subscriber units 60-1 through 60-3 communicate with a
centrally located base station 65. Each subscriber unit 60 is
equipped with an antenna 70 configured according to the present
invention. The subscriber units 60 are provided with wireless data
and/or voice services by the system operator and can connect
devices such as, for example, laptop computers, portable computers,
personal digital assistants (PDAs) or the like through base station
65 (including the antenna 68) to a network 75, comprising the
public switched telephone network (PSTN), a packet switched
computer network such as the Internet, a public data network or a
private intranet. The base station 65 communicates with the network
75 over any number of different available communications protocols
such as primary rate ISDN, or other LAPD based protocols such as
IS-634 or V5.2, or even TCP/IP if the network 75 is a packet based
Ethernet network such as the Internet. The subscriber units 60 may
be mobile in nature and may travel from one location to another
while communicating with the base station 65. As the subscriber
units leave one cell and enters another, the communications link is
handed off from the base station of the exiting cell to the base
station of the entering cell.
[0034] FIG. 1 illustrates one base station 65 and three mobile
subscriber units 60 in a cell 50 by way of example only and for
ease of description of the invention. The invention is applicable
to systems in which there are typically many more subscriber units
communicating with one or more base stations in an individual cell,
such as the cell 50.
[0035] It is also to be understood by those skilled in the art that
FIG. 1 represents a standard cellular type communications system
employing signaling schemes such as a CDMA, TDMA, GSM or others, in
which the radio channels are assigned to carry data and/or voice
between the base stations 65 and subscriber units 60. In one
embodiment, FIG. 1 is a CDMA-like system, using code division
multiplexing principles such as those defined in the IS-95B
standards for the air interface. It is further understood by those
skilled in the art that the various embodiments of the present
invention can be employed in other wireless communications systems
operating under various communications protocols, including the
IEEE 802.11 standards and the Bluetooth standards.
[0036] In one embodiment of the cell-based system, the mobile
subscriber units 60 employ an antenna 70 that provides directional
reception of forward link radio signals transmitted from the base
station 65, as well as directional transmission of reverse link
signals (via a process called beam forming) from the mobile
subscriber units 60 to the base station 65. This concept is
illustrated in FIG. 1 by the example beam patterns 71 through 73
that extend outwardly from each mobile subscriber unit 60 more or
less in a direction for best propagation toward the base station
65. By directing transmission more or less toward the base station
65, and directively receiving signals originating more or less from
the location of the base station 65, the antenna apparatus 70
reduces the effects of intercell interference and multipath fading
for the mobile subscriber units 60. Moreover, since the antenna
beam patterns 71, 72 and 73 extend outward in the direction of the
base station 65 but are attenuated in most other directions, less
power is required for transmission of effective communications
signals from the mobile subscriber units 60-1, 60-2 and 60-3 to the
base station 65. Thus the antennas 70 provide increased gain when
compared with an isotropic radiator.
[0037] One antenna array embodiment providing a directive beam
pattern and further to which the teachings of the present invention
can be applied, is illustrated in FIG. 2. The FIG. 2 antenna array
100 comprises a four-element circular array provided with four
antenna elements 103. A single-path network feeds each of the
antenna elements 103. The network comprises four fifty-ohm
transmission lines 105 meeting at a junction 106, with a 25-ohm
transmission line 107. Each of the antenna feed lines 105 has a
switch 108 interposed along the feed line. In FIG. 1, each switch
108 is represented by a diode, although those skilled in the art
recognize that other switching elements can be employed in lieu of
the diodes, including the use of a single-pole-double-throw (SPDT)
switch. In any case, each of the antenna elements 103 is
independently controlled by its respective switch 108. A 35-ohm
quarter-wave transformer 110 matches the 25-ohm transmission line
107 to the 50-ohm transmission lines 105.
[0038] In operation, typically two adjacent antenna elements 103
are connected to the transmission lines 105 via closing of the
associated switches 108. Those elements 103 serve as active
elements, while the remaining two elements 103 for which the
switches 108 are open, serve as reflectors. Thus any adjacent pair
of the switches 108 can be closed to create the desired antenna
beam pattern. The antenna array 100 can also be scanned by
successively opening and closing the adjacent pairs of switches
108, changing the active elements of the antenna array 100 to
effectuate the beam pattern movement. In another embodiment of the
antenna array 100, it is also possible to activate only one
element, in which case the transition line 107 has a 50-ohm
characteristic impedance and the quarter-wave transformer 110 is
unnecessary.
[0039] Another antenna design that presents an inexpensive,
electrically small, low loss, low cost, medium directivity,
electronically scanable antenna array is illustrated in FIG. 3.
This antenna array 130 includes a single excited antenna element
surrounded by electronically tunable passive elements that serve as
directors or reflectors as desired. The exemplary antenna array 130
includes a single central active element 132 surrounded by five
passive reflector-directors 134 through 138. The
reflector-directors 134-138 are also referred to as passive
elements. In one embodiment, the active element 132 and the passive
elements 134 through 138 are dipole antennas. As shown, the active
element 132 is electrically connected to a fifty-ohm transmission
line 140. Each passive element 134 through 138 is attached to a
single-pole double throw (SPDT) switch 160. The position of the
switch 160 places each of the passive elements 134 through 138 in
either a directive or a reflective state. When in a directive
state, the antenna element is virtually invisible to the radio
frequency signal and therefore directs the radio frequency energy
in the forward direction. In the reflective state the radio
frequency energy is returned in the direction of the source.
[0040] Electronic scanning is implemented through the use of the
SPDT switches 160. Each switch 160 couples its respective passive
element into one of two separate open or short-circuited
transmission line stubs. The length of each transmission line stub
is predetermined to generate the necessary reactive impedance for
the passive elements 134 through 138, such that the directive or
reflective state is achieved. The reactive impedance can also be
realized through the use of an application-specific integrated
circuit or a lumped reactive load.
[0041] When in use, the antenna array 130 provides a fixed beam
directive pattern in the direction identified by the arrowhead 164
by placing the passive elements 134, 137 and 138 in the reflective
state while the passive elements 135 and 136 are switched to the
directive state. Scanning of the beam is accomplished by
progressively opening and closing adjacent switches 160 in the
circle formed by the passive elements 134 through 138. An
omnidirectional mode is achieved when all of the passive elements
134 through 138 are placed in the directive state.
[0042] As will be appreciated by those skilled in the art, the
antenna array 130 has N operating directive modes, where N is the
number of passive elements. The fundamental array mode requires
switching all of the N passive elements to the directive state to
achieve an omnidirectional far-field pattern. Progressively
increasing directivity can be achieved by switching from one to
approximately half the number of passive elements into the
reflective state, while the remaining elements are directive.
[0043] FIG. 4 illustrates an antenna array 198 comprising six
vertical monopoles 200 arranged at an approximately equal radius
(and having approximately equal angular spacing there between),
from a center element 202. The center element is the active
element, in the transmitting mode, as indicated by the alternating
input signal referred to with reference character 206. According to
the antenna reciprocity theorem, the active element 202 functions
in a reciprocal manner for signals transmitted to the antenna array
198. The passive elements 200 shape the radiation pattern from (or
to) the active element 202 by selectively providing reflective or
directive properties at their respective location. The
reflective/directive properties or a combination of both is
determined by the setting of the variable reactance element 204
associated with each of the passive elements 200. When the passive
elements 200 are configured to serve as directors, the radiation
transmitted by the active element 202 (or received by the active
element 202 in the receive mode) passes through the ring of passive
elements 200 to form an omnidirectional antenna beam pattern. When
the passive elements 200 are configured in the reflective mode, the
radio frequency energy transmitted from the active element 202 is
reflected back toward the center of the antenna ring. Generally, it
is known that changing the resonant length causes an antenna
element to become reflective when the element is longer than the
resonant length, (wherein the resonant length is defined as
.lambda./2 or .lambda./4 if a ground plane is present below the
antenna element) or directive/transparent when the element is
shorter than the resonant length. A continuous distribution of
reflectors among the passive elements 200 collimates the radiation
pattern in the direction of those elements configured as
directors.
[0044] As shown in FIG. 4, each of the passive elements 200 and the
active element 202 are oriented for vertical polarization of the
transmitted or received signal. It is known to those skilled in the
art that horizontal placement of the antenna elements results in
horizontal signal polarization. For horizontal polarization, the
active element 202 is replaced by a loop or annular ring antenna
and the passive elements 202 are replaced by horizontal dipole
antennas.
[0045] According to the teachings of the present invention, the
energy passing through the directive configured passive elements
200 can be further shaped into a more directive antenna beam. As
shown in FIG. 5, the beam is shaped by placement of an annular
dielectric substrate 210 around the antenna array 198. The
dielectric substrate is in the shape of a ring with an outer band
defining an interior aperture, with the passive elements 200 and
the active element 202 disposed within the interior aperture. The
dielectric substrate 210 is a slow wave structure having a lower
propagation constant than air. As a result, the portion of the
transmitted wave (or the received wave in the receive mode) that
contacts the dielectric substrate 210 is guided and slowed relative
to the free space portion of the wave. As a result, the radiation
pattern in the elevation direction narrows (the elevation energy is
attenuated) and the radiation is focused toward the azimuth
direction. Thus the antenna beam pattern gain is increased. The
slow-wave structure essentially guides the power or radiated energy
along the dielectric slab to form a more directive beam. In one
embodiment, the radius of the dielectric substrate 210 is at least
a half wavelength. As is known to those skilled in the art, a slow
wave structure can take many forms, including a dielectric slab, a
corrugated conducting surface, conductive gratings or any
combination thereof.
[0046] Typically, the variable reactance elements 204 are tuned to
optimize operation of the passive elements 200 with the dielectric
substrate 210. For a given operational frequency, once the optimum
distance between the passive elements 200 and the circumference of
the interior aperture of the dielectric substrate 210 has been
established, this distance remains unchanged during operation at
the given frequency.
[0047] FIG. 6 illustrates the dielectric substrate 210 along cross
section 6-6 of FIG. 5. The dielectric substrate 210 includes two
tapered edges 218 and 220. A ground plane 222 below the dielectric
substrate 210 can also be seen in this view. Both of these tapered
edges 218 and 220 edges ease the transition from air to substrate
or vice versa. Abrupt transitions cause reflections of the incident
wave, which, in this situation, reduces the effect of the slow-wave
structure.
[0048] Although the tapers 218 and 220 are shown of unequal length,
those skilled in the art will recognize that a longer taper
provides a more advantageous transition between the free space
propagation constant and the dielectric propagation constant. The
taper length is also dependent upon the space available for the
dielectric slab 210. Ideally, the tapers should be long if
sufficient space is available for the dielectric substrate 210.
[0049] In one embodiment, the height of the dielectric substrate
210 is the wavelength of the received or transmitted signal divided
by four (i.e., .lambda./4). In an embodiment where the ground plane
222 is not present, the height of the dielectric slab 210 is
.lambda./2. The wavelength .lambda., when considered in conjunction
with the dielectric substrate 210, is the wavelength in the
dielectric, which is always less than the free space wavelength.
The antenna directivity is a monotonic function of the dielectric
substrate radius. A longer dielectric substrate 210 provides a
gradual transition over which the radio frequency signal passes
from the dielectric substrate 210 into free space (and vice versa
for a received wave). This allows the wave to maintain collimation,
increasing the antenna array directivity when the wave exits the
dielectric substrate 210. As known by those skilled in the art,
generally, the antenna directivity is calculated in the far field
where the wave front is substantially planar.
[0050] In one embodiment, the passive elements 200, the active
element 202 and the dielectric substrate 210 are mounted on a
platform or within a housing for placement on a work surface. Such
a configuration can be used with a laptop computer, for example, to
access the Internet via a CDMA wireless system or to access a
wireless access point, with the passive elements 200 and the active
element 202 fed and controlled by a wireless communications devices
in the laptop. In lieu of placing the antenna elements 200 and 202
and the dielectric substrate 210 in a separate package, they can
also be integrated into a surface of the laptop computer such that
the passive elements 200 and the active element 202 extend
vertically above that surface. The dielectric substrate 210 can be
either integrated within that laptop surface or can be formed as a
separate component for setting upon the surface in such a way so as
to surround the passive elements 200. When integrated into the
surface, the passive elements 200 and the active element 202 can be
foldably disposed toward the surface when in a folded state and
deployed into a vertical state for operation. Once the passive
elements 200 and the active element 202 are vertically oriented,
the separate dielectric slab 210 can be fitted around the passive
elements 200.
[0051] The dielectric substrate 210 can be fabricated using any
low-loss dielectric material, including polystyrene, alumina,
polyethylene or an artificial dielectric. As is known by those
skilled in the art, an artificial dielectric is a volume filled
with hollow metal spheres that are isolated from each other.
[0052] FIG. 7 illustrates an antenna array 230, including a
corrugated metal disk 250 surrounding the passive antenna elements
200. The corrugated metal disk 250, which offers similar
gain-improving functionality as the dielectric substrate 210 in
FIG. 5, comprises a plurality of circumferential mesas 252 defining
grooves 254 there between. FIG. 8 is a view through section 8-8 of
FIG. 7. Note that the innermost mesa 252A includes a tapered
surface 256. Also, the outermost mesas 252B and 252C include
tapered surfaces 258 and 260, respectively. As in the FIG. 5
embodiment, the tapers 256 and 258 provide a transition region
between free space and the propagation constant presented by the
corrugated metal disk 250. Like the dielectric substrate 210, the
corrugated metal disk 250 serves as a slow-wave structure because
the grooves 254 are approximately a quarter-wavelength deep and
therefore present an impedance to the traveling radio frequency
signal that approximates an open, i.e., a quarter-wavelength in
free space. However, because the notches do not present precisely
an open circuit, the impedance causes bending of the traveling wave
in a manner similar to the bending caused by the dielectric
substrate 210 of FIG. 5. If the grooves 254 were to provide a
perfect open, no radio frequency energy would be trapped by the
groove and there would be no bending of the wave. The key to
successful utilization of the FIG. 7 embodiment is the trapping of
the radio frequency wave. When the grooves 254 are shallow, they
release the wave and thus the contouring (i.e., the location of the
mesas and grooves) controls the location and degree to which the
wave is allowed to radiate to form a collimated wave front. For
example, if the grooves were radially oriented, the wave would
simply travel along the grooves and could not be controlled.
Although the FIGS. 7 and 8 embodiments illustrate only three
grooves or notches, it is known by those skilled in the art that
additional grooves or notches can be provided to further control
the traveling radio frequency wave and improve the directivity of
the antenna in the azimuth direction.
[0053] FIG. 9 illustrates an antenna array 258 representing another
embodiment of the present invention, including a ground plane 260,
the previously discussed active element 202 and the passive
elements 200. Additionally, FIG. 9 illustrates a plurality of
parasitic conductive gratings 262. In the embodiment of FIG. 9, the
parasitic conductive gratings 262 are shown as spaced apart from
and along the same radial lines as the passive elements 200. In a
sense, the antenna array 258 of FIG. 9 is a special case of the
antenna array 230 of FIG. 7. The height of the circumferential
mesas 252 is represented by the position of the parasitic
conductive gratings 262. The taper of the outer mesas 252B and 252C
in FIG. 8 is repeated by tapering the parasitic conductive gratings
262 in the direction away from the center element 202.
[0054] FIG. 10 illustrates the antenna array 258 in cross section
along the lines 10-10. Exemplary lengths for the passive elements
200 and the active element 202 are also shown in FIG. 10. Further,
exemplary height and spacing between the parasitic conductive
gratings 262 at 1.9 GHz are also set forth. Generally, the spacing
is about 0.9.lambda. to 0.28.lambda.. The spacing between the
active element 202, the passive elements 200, and the plurality of
parasitic conductive gratings 262 are generally tied to the height
of each element. If the passive elements 200 and the plurality of
parasitic conductive gratings 262 are a resonant length, the
element simply resonates and thereby retains the received energy.
Some energy may spill over to neighboring elements. If the element
is shorter than a resonant length, then the impedance of the
element causes it to act as a forward scatterer due to the imparted
phase advance. Scattering is the process by which a radiating wave
strikes an obstacle, and then re-radiates in all directions. If the
scattering is predominant in the forward direction of the traveling
wave, then the scattering is referred to as forward scattering. If
the element is longer than a resonant length, the resulting phase
retardation interacts with the original traveling wave thereby
reducing or even canceling the forward traveling radiation. As a
result, the energy is scattered backwards. That is, the element
acts as a reflector. In the FIG. 9 embodiment, the plurality of
parasitic conductive gratings 262 can be either shorted to the
ground plane 260 or adjustably reactively loaded, where the loading
effectively adjusts the effective length of any one of the
plurality of parasitic conductive gratings 262 causing the
parasitic conductive grating 262 to have a length equal to, less
than or greater than the resonant length, with the resulting
directive or reflective effects as discussed above. Providing this
controllable reactive feature provides the ability to vary the
degree of directivity or beam pattern width as desired.
[0055] It should also be noted that in the FIG. 9 embodiment the
ground plane 260 is pentagonal in shape. In another embodiment, the
ground plane can be circular. In one embodiment, the number of
facets in the ground plane 260 is equal to the number of passive
elements. As in the embodiments of FIGS. 5 and 7, the plurality of
gratings or parasitic conductive elements 262 serve to slow the
radio frequency wave and thus improve the directivity in the
azimuth direction. Adding more gratings causes further reductions
in the RF energy in the elevation direction. Note that the beam
pattern produced by the antenna array 258 includes five individual
and highly directive lobes when each of the passive elements 200 is
placed in the directive state. When two adjacent passive elements
200 are placed in a directive state, the highly directive lobe is
formed in a direction between the two directive elements, due to
the addition of the energy of each lobe. When all passive elements
200 are placed in a directive state simultaneously, an
omni-directional pancake pattern (i.e., relatively close to the
plane of the ground plane 260) is created.
[0056] As compared with the grooves 254 of FIG. 7, the parasitic
conductive gratings 262 of FIG. 9 have sharper resonance peaks and
therefore are very efficient in slowing the traveling RF wave.
However, as also discussed in conjunction with FIG. 7, the
parasitic conductive gratings 262 are not spaced at precisely the
resonant frequency. Instead, a residual resonance is created that
causes the slowing effect in the radio frequency signal.
[0057] The antenna array 270 of FIG. 11 includes the elements of
FIG. 9, with the addition of a plurality of interstitial parasitic
elements 272 between the parasitic conductive gratings 262, to
further guide and shape the radiation pattern. The interstitial
parasitic elements 272 are shorted to the ground plane 260 and
provide additional refinement of the beam pattern. The interstitial
parasitic elements 272 are placed experimentally to afford one or
more of the following objectives: reducing the ripple in the
omnidirectional pattern, adding intermediate high-gain beam
positions when the array is steered through the resonant
characteristic of the parasitic elements 200, reducing undesirable
side lobes and improving the front to back power ratio.
[0058] In one embodiment, an antenna constructed according to the
teachings of FIG. 11, has a peak directivity of 8.5 to 9.5 dBi over
a bandwidth of about thirty percent. By electronically controlling
the reactance of each passive element 200, this high-gain antenna
beam can also be steered. When all of the passive elements 200 are
in the directive mode, an omnidirectional beam substantially in the
azimuth plane is formed. In the omnidirectional mode, the peak
directivity was measured at 5.6 to 7.1 (dBi) over the same
frequency band as the directive mode. Thus, the FIG. 11 embodiment
provides both a high-gain omnidirectional pattern and a high-gain
steerable beam pattern. For an antenna operative at 1.92 GHz in one
embodiment, the approximate height of the interstitial parasitic
elements 272 is 1.5 inches and the distance from the active element
202 to the outer interstitial parasitic elements 272 is
approximately 7.6 inches.
[0059] The antenna array of FIG. 12 is derived from FIG. 9, where
an axial row of the parasitic conductive gratings 262 and one
passive element 200 are integrated into or disposed on a dielectric
substrate or printed circuit board 280. Note that in the FIG. 9
embodiment, the passive elements 200 and the parasitic conductive
gratings 262 are fabricated individually. The passive elements 200
are separated from the ground plane 260 by an insulating material
and conductively connected to the reactance control elements
previously discussed. The parasitic conductive gratings 262 are
shorted directly to the ground plane 260 or controllably reactively
loaded as discussed above. Thus the process of fabricating the FIG.
9 embodiment is time intensive. The FIG. 12 embodiment is therefore
especially advantageous because the parasitic conductive gratings
262 and the passive elements 200 are printed on or etched from a
dielectric substrate or printed circuit board material. This
process of integrating and grouping the various antenna elements as
shown, provides additional mechanical strength and improved
manufacturing precision with respect to the height and spacing of
the elements. Due to the use of a dielectric material between the
various antenna elements, the FIG. 12 embodiment can be considered
a hybrid between the dielectric substrate embodiment of FIG. 5 and
the conductive grating embodiment of FIG. 9. In particular, the
dielectric substrate 280 smoothes the discrete resonant properties
of the parasitic conductive gratings 262, thereby reducing the
formation of gain spikes in the frequency spectrum of the
operational bandwidth.
[0060] FIG. 13 illustrates another process for fabricating the
antenna array 258 of FIG. 9 and the antenna array 270 of FIG. 11.
In the FIG. 13 process, the parasitic conductive gratings 262 (and
the interstitial parasitic elements 272 in FIG. 11) are stamped
from the ground plane 260 and then bent upwardly to form the
parasitic conductive gratings 262 (and the interstitial parasitic
elements 272 in FIG. 11). This process is illustrated in greater
detail in the enlarged view of FIG. 14. In one embodiment, the
parasitic conductive gratings 262 and the interstitial parasitic
elements 272 are formed by removing a U-shaped region of material
from the ground plane 260 such that a deformable joint is formed
along an edge of the U-shaped opening where the ground plane
material has not been removed. The parasitic conductive gratings
262 and the interstitial parasitic elements 272 are then formed by
bending the ground plane material along the joint and out of the
plane of the ground plane 260. The void remaining after removing
the U-shaped region of the ground plane 260 is referred to by
reference character 274. It has been found that the void 274 does
not significantly affect the performance of the antenna array 258
(FIG. 9) and 270 (FIG. 11). In the FIG. 13 embodiment, the active
element 202 and the passive elements 200 are formed on a separate
metallic disc 280, which is attached to the ground plane 260 using
screws or other fasteners 282.
[0061] FIG. 15 is a perspective schematic view of an antenna 300
constructed according to the teachings of another embodiment of the
present invention, depicted with reference to a coordinate system
301. The antenna 300 radiates a substantial percentage of the
transmitted energy in an XY plane, where the plane is perpendicular
to the active element 202 and referred to as the horizon. In the
receiving mode the antenna 300 receives a substantial percentage of
the received energy in the same XY plane. Generally, the antenna
300 is more directive along the horizon than the embodiments
described above. Advantageously, the ground plane of the antenna
300 is smaller than the ground plane of the embodiments described
above, thus requiring a smaller space envelope. These features will
be discussed further below.
[0062] In the top view of FIG. 16, the antenna 300 comprises a
plurality of segments 302 formed from antenna elements that are
controllable to reflect or direct the signal emitted from the
active element 202 located at a hub 304. In the receiving mode, the
antenna elements reflect or direct the received signal. As is known
to those skilled in the art, the reflective or directive property
is a function of the antenna element effective length as related to
the operating frequency. Thus controlling the effective element
length, for example, by changing the element's physical length or
by the switchable connection of an impedance to the element,
achieves the reflective or directive state.
[0063] Those skilled in the art recognize that more or fewer
segments 302, and thus more or fewer antenna elements, can be
employed to produce other desired radiation patterns, including
more directive antenna patterns, than achievable with the six
segments 302 of FIG. 16. The segments of FIG. 16 are shown as
spaced at 60.degree. intervals, but the spacing is also selectable
based on the desired radiation pattern.
[0064] Two oppositely disposed segments 302 are illustrated in FIG.
17. Each segment 302 comprises a passive dipole 308, further
comprising an upper segment 308A and a lower segment 308B. The
remaining segments 302, not illustrated in FIG. 17, are similarly
constructed. The lower segment 308B is contiguous with a ground
plane 312 and is thus formed from a shaped region of the ground
plane 312. In one embodiment the ground plane 312 is formed from
printed circuit board material e.g., a dielectric substrate with a
conductive layer disposed thereon.
[0065] By placing each of the passive dipoles 308 in a reflective
or a directive state, the antenna beam can be formed in a specific
azimuth direction relative to the active element 202. Beam scanning
is accomplished by progressively placing each of the passive
dipoles 308 into a directive/reflective state. An omnidirectional
radiation pattern is achieved when all of the passive dipoles are
operated in a directive state.
[0066] The upper segment 308A operates as a switched parasitic
element, similar to the passive elements 200 described above,
loaded through a schematically-illustrated switch 310 and in
conjunction with the lower segment 308B, forms a dipole operative
as a director (a forward scattering element) or as a reflector in
response to the impedance load applied through the switch 310. A
separate controller (not shown) is operative to determine the state
of the passive dipole (e.g., reflective or directive) in response
to user-supplied inputs or in response to known signal detection
and analysis techniques for controlling the antenna parameters to
provide the highest quality received or transmitted signal. Such
techniques conventionally include determining one or more signal
metrics of the transmitted or received signal and in response
thereto modifying one or more antenna characteristics to improve
the transmitted or received signal metric.
[0067] The upper segment 308A is fed as a monopole element, and the
lower segment 308B is part of a ground structure that mirrors the
upper segment 308A. But because the lower segment 308B is grounded,
the circuit equivalent of the passive dipole 308 is a monopole over
a ground plane. The radiation characteristics of the passive dipole
308 resemble a dipole because the lower segment 308B resonates with
the upper segment 308A. Thus the passive dipole is fed as
space-feed element, such that the upper and lower segments 308A and
308B intercept the radio frequency wave and reradiate it like a
passive dipole. Since the lower segment 308B is a part of the
ground plane 312, balanced loading of the dipole element 308 is not
necessary and a balun is not required.
[0068] The switchable loading can be a simple impedance, yet the
passive dipole 308 radiates with symmetry like a conventional
dipole. Advantageously, using the passive dipole 308 provides the
higher gain of a dipole, and also the symmetry creates radiation
toward the horizon, rather than tilted away from the horizon. The
impedance loading can be treated as an extension of the upper
segment 308A. If the loading is inductive, the effective length of
308A becomes longer, and the reverse is true for a capacitive
loading. Inductive loading makes the combination of the upper and
the lower segments 308A and 308B operate as a reflector.
Conversely, the combination operates as a director in response to
capacitive loading.
[0069] FIG. 18 illustrates the switch 310 and associated components
in greater detail. Although illustrated as a mechanical switch,
those skilled in the art recognize that the switch 310 can be
implemented by a semiconductor device (a metal-oxide semiconductor
field effect transistor) or a MEMS (microelectomechanical systems)
switch. As illustrated in FIG. 18, the switch 310 switchably
connects impedances Z1 and Z2 to the upper segment 308A. Both of
the impedances Z1 and Z2 are connected to ground at their
respective non-switched terminals. Although the specific values for
the impedances Z1 and Z2 are selected based on one or more desired
antenna operating parameters (e.g., gain, operating frequency,
bandwidth, radiation pattern shape), generally one of the impedance
values (Z1 for example) is substantially a capacitive impedance and
the other, Z2, is substantially an inductive impedance. The
impedances can be provided by lumped or distributed circuit (e.g.,
a delay line) elements. In other embodiments, the values for Z1 and
Z2 can both be capacitive (or both inductive) with one value more
capacitive (or inductive) than the other to achieve the desired
performance parameters. In other embodiments more than two
impedances can be switchably introduced into the upper segment 308A
to provide other desired performance characteristics.
[0070] In an embodiment where Z1 is substantially capacitive, the
associated passive dipole 308 operates as a director when the
switch 310 is in a position to connect the upper segment 308A to
ground via Z1. When connected to a substantially inductive Z2, the
passive dipole 308 operates as a reflector. In either case, current
flow induced in the upper segment 308A and the lower segment 308B
by the received or transmitted radio frequency signal produces a
symmetrical dipole effect, resulting in substantial energy directed
proximate the XY plane. Since the passive dipole 308 form more
directive horizon beams than a monopole element above a finite
ground plane (i.e., the embodiments described above) the antenna
300 exhibits better gain along the horizon than those antenna
embodiments described above.
[0071] It has been determined, according to the present invention,
that optimum antenna gain is achieved when the length H in FIG. 17
is between about 0.25.lambda. and slightly less than 0.5.lambda. at
the operational frequency. The antenna gain may be reduced for
other values of H outside this range.
[0072] With continuing reference to FIG. 17, in one embodiment a
region 314 comprises a matching element (not shown) for connecting
the active element 202 to a source providing the radio frequency
signal to be transmitted from the active element 202 and/or to a
receiver to which the active element 202 supplies a received
signal.
[0073] Use of the passive dipoles 308 in lieu of the passive
elements 200 and the parasitic conductive gratings 262 as described
in the embodiments above, provides improved horizon directivity for
the antenna 300, pointing the antenna beam substantially along the
horizon. In one example, the improvement is about 4 dB. Since the
passive dipoles 308 comprise physically distinct upper and lower
segments 308A and 308B, they provide better directive
characteristics than the monopole elements (i.e., the passive
elements 200 and the parasitic conductive gratings 262) that
operate in a dipole mode in conjunction with an image element below
the ground plane. Theoretically, an infinite ground plane produces
a perfect image element. In practice, the ground plane 260 (see
FIG. 9, for example) is finite and thus the image elements are not
ideal, resulting in reduced directivity in the direction of the
horizon. Use of the passive dipoles 308 improves the directivity of
the antenna 300.
[0074] Returning to FIG. 15, a parasitic directing element 320
(also referred to as a short-circuited dipole) is disposed in
substantially the same vertical plane as each dipole element 308
and connected to the ground plane 312 via a conductive arm 322. The
parasitic directing elements 320, which are typically shorter than
a half wavelength at the operating frequency of the antenna 300,
operate as forward scattering elements, directing the transmitted
signal toward the horizon. Since the arm 322 is orthogonal to the
polarization of the signal transmitted from the active element 202,
the arm 322 is not coupled to the signal and thus does not affect
antenna operation. Therefore, in another embodiment the arm
material comprises a dielectric. The parasitic directing elements
320 are not necessarily required for operation of the antenna 300,
but advantageously provide additional directive effects with regard
to propagation of the signal proximate the horizon.
[0075] In other embodiments an antenna constructed according to the
teachings of the present invention comprises more or fewer passive
dipoles 308 and parasitic directing elements 320 as determined by
the desired radiation pattern. In still another embodiment the
number of passive dipoles 308 is not necessarily equal to the
number of parasitic directing elements 320.
[0076] Advantageously, the lower segment 308B, the ground plane 312
and the parasitic directing elements 320 on one spoke 302 comprise
a unitary structure or a unitary shaped ground plane. In another
embodiment the elements can be separately formed and connected by
conductive wires or solder joints.
[0077] With reference to FIG. 15, a ground plane 330 surrounds the
active element 202 and is connected to the ground plane 312. Note
in the illustrated embodiment the ground plane 330 is
advantageously smaller than the ground planes illustrated in the
embodiments illustrated above. However the antenna 300 provides
improved directivity proximate the XY plane (the horizon) due to
the use of the dipole elements 308, rather than relying on image
elements as in the antenna 258 of FIG. 9. In another embodiment the
ground plane 330 is not required. In yet another embodiment, the
ground plane 330 can be shaped to include the function of the
ground plane 312.
[0078] Both of the ground planes 312 and 330 can be scaled in
relation to the operative frequency of the antenna 300. In an
embodiment where the ground plane 312 and/or 330 comprises a
dielectric substrate and a conductive layer disposed thereon,
electronic circuit elements can be mounted on the substrate and
operative to control operation of the antenna elements and to feed
or receive the radio frequency signal to/from the active element
202. To mount the electronic circuit elements on the substrate, a
region of the substrate is isolated from the ground conductor and
conductive interconnections are formed on the isolated region by
patterning and etching techniques. Such mounting techniques are
know in the art. In particular, the switches 310 are disposed on
the ground planes 312 and/or 330. Because the electronic circuit
elements do not scale to the operational frequency of the antenna
300, a larger surface area than required for the operational
frequency may be required for mounting the circuit elements.
[0079] FIG. 19 illustrates another embodiment according to the
teachings of the present invention, comprising directive parasitic
elements 340 (also referred to as short circuit dipole elements)
disposed radially outward and electrically connected to the
directive parasitic elements 320 via an arm 342. This embodiment
provides additional gain along the horizon. Although FIG. 19
illustrates only two such directive parasitic elements 340, in a
preferred embodiment each spoke 302 carries a directive parasitic
element 340.
[0080] FIG. 20 illustrates another embodiment of an antenna 345
comprising a ring 346 physically connected to and supporting the
parasitic directive elements 320, in lieu of the arms 322
illustrated in FIG. 15. The material of the ring 346 comprises a
conductor or a dielectric. Use of the ring 346 also provides a
support mechanism for the placement of interstitial parasitic
elements (not shown in FIG. 20) between adjacent parasitic
directing elements 320.
[0081] In another embodiment, an antenna comprises an inner core
segment (comprising the active element 202 and the passive dipoles
308) and a removable outer segment comprising the parasitic
directive elements 320 supported by the ring 346. Thus if the gain
provided by the inner core segment is sufficient the outer segment
is not required and the antenna space requirements are minimized.
If additional directivity is desired, the outer segment is easily
and conveniently positioned around the inner core segment.
[0082] In the above embodiments the active element 202, the dipole
elements 308 and the parasitic directing elements 320 and 340 are
illustrated as simple linear elements. As can be appreciated by
those skilled in the art, other element shapes can be used in place
of the linear elements to provide element resonance and reflection
characteristics over a wider bandwidth or at two or more resonant
frequencies. Several exemplary element shapes are illustrated in
FIGS. 21A-21D. An element 360 of FIG. 21A resonates at two
different frequencies as determined by the two height dimensions,
h1 and h2, where h1 is the longer dimension and therefore a region
361 resonates at a lower frequency than a region 362. Additional
resonant frequencies can be obtained by providing additional
resonant segments within the element 360. A triangular element 364
of FIG. 21B provides broadband resonance due to the multiple
resonant currents that can be established in multiple length paths
365 and 366 (only two exemplary paths are illustrated) between an
apex 367 and a base 368. In another embodiment the apex angle and
the side lengths can be adjusted to provide log-periodic
performance. A fat element such as an element 369 of FIG. 21C
provides broader bandwidth performance than the relatively narrower
elements described above. A cylindrical element 372 of FIG. 21D is
a three-dimensional structure, as compared with the two-dimensional
structures of FIG. 20, for example, capable of providing multiple
resonant paths as the signal traverses reflective paths, including
one of the exemplary paths 373 and 374, as illustrated. Each of the
illustrated elements and any other known monopole-type elements can
be substituted for the upper segment 308A, and/or the lower segment
308B and/or the parasitic directing elements 320 and 340.
[0083] By taking advantage of known harmonic relationships between
signal frequencies, the antenna 300 of FIG. 15 can provide multiple
resonant frequency operation. It is known that all antennas and
antenna arrays exhibit multiple resonances. In particular, dipole
elements resonate when the length is near a half wavelength of the
operative frequency, and integer multiples thereof. Optimum array
elements spacing is similarly harmonically related. Thus the
spacing between the active element 202 and the passive dipoles 308,
and the length of the passive dipoles 308 can be selected, in one
embodiment, so that the antenna 300 resonates at two
nearly-harmonically related frequencies, such as 5.25 GHz as
governed by the IEEE 802.11a standard and 2.45 GHz as governed by
the IEEE 802.11b standard. See for example the commonly owned
patent application entitled, "A Dual Band Phased Array Antenna
Employing Spatial Second Harmonics," filed on Nov. 8, 2002 and
assigned application number 10/292,384 (Attorney's docket number
TAN01-61).
[0084] FIG. 22 illustrates an antenna 400 constructed according to
another embodiment of the present invention, comprising
substantially identical sections 402A-402D and a center dual
section 406. As illustrated in FIG. 23, the center dual section 406
comprises the ground plane 312 electrically connected to the lower
segments 308B. The switch 310 controls operation of the upper
segments 308A via the switch 310. Like the upper segments 308A, the
active element 202 is physically connected to the center element
202 but insulated from the ground plane conductor. Electronic
components (not shown) are mounted on the center dual section 406
for providing radio frequency signals to and receiving radio
frequency signals from the active element 202 and for controlling
operation of the switches 310. The center dual section 406 and the
sections 402A-402D are joined by a support member 407. In another
embodiment (not shown) the antenna comprises two support members,
including an upper support member disposed proximate an upper
surface 405 of the ground plane 312, and a lower support member
disposed proximate a lower surface 407. The upper and lower support
members join the center dual section 406 and the sections
402A-402D. The material of the support member 407 comprises a
conductive, dielectric or composite material (e.g., a conductive
material disposed on a dielectric substrate).
[0085] FIG. 24 illustrates the section 402A, comprising a ground
plane 410 electrically connected to the ground plane 312 when the
sections 402A-402D and the center dual section 406 are assembled to
form the antenna 400. The ground plane 410 is electrically
connected to the lower segments 308B.
[0086] As can be seen, an antenna constructed according to the
various embodiments of the invention maximizes the effective
radiated and/or received energy along the horizon. The antenna
accomplishes the gain improvement by the use of a ring of passive
dipoles. Also, by controlling certain characteristics of the
passive dipoles the antenna is scanable in the azimuth plane. By
providing higher antenna gain for a wireless network, various
interference problems are minimized, the communications range is
increased, and higher data rate and wider bandwidth signals can be
accommodated.
[0087] While the invention has been described with reference to a
preferred embodiment, it will be understood by those skills in the
art that various changes may be made and equivalent elements may be
substituted for elements thereof without departing from the scope
of the present invention. In addition, modifications may be made to
adapt a particular situation more material to teachings of the
present invention without departing from the essential scope
thereof. Therefore, it is intended that the invention not be
limited to the particular embodiment disclosed at the best mode
contemplated for carrying out this invention, but that the
invention include all embodiments falling within the scope of the
appended claims.
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